1. Field of the Invention
This invention relates generally to heat transfer devices and more particularly to vapor chamber cooking devices providing an isothermal cooking surface under changing conditions of thermal load and which is reliably protected from overpressure conditions.
2. Description of the Prior Art
Among a number of devices which have been employed for cooking is the directly heated griddle in which heat is applied directly to the underside of a flat metal cooking surface. Such griddles may vary in surface area from the small ones used in the home to the very large ones used in professional food preparation establishments such as restaurants, hotels and other mass producers of cooked foods. In most applications, the only temperature control afforded is through adjustment of the source of heat by the operator of the griddle. Such an adjustment requires discovery of an undesired temperature condition on the surface of the griddle, a trial adjustment of the heat source, an observation of how well the trial adjustment succeeds and, usually, a repetition of the whole process until the desired temperature is obtained. Moreover, the temperature of the cooking surface of a directly heated griddle is dependent upon the thermal load and may become highly nonuniform as a result of significant changes in thermal load conditions. For example, the griddle may be in equilibrium at the temperature required to prepare two hamburgers; however, the addition of five more frozen hamburgers will upset the temperature equilibrium to such an extent that it becomes necessary to apply more source heat for the purpose of offsetting the changed thermal load conditions. In the application of source heat to the griddle, hot spots are developed due to uneven distribution of the source heat and inhomogeneous thermal conductivity of the flat metal surface itself. Thus, directly heated devices such as griddles have poor temperature regulation and control, require a great amount of monitoring, and develop unpredictable hot spots as a result of changed conditions of thermal load.
Another cooking device commonly employed by restaurants, cafeterias, hotels, and other institutions where mass processing of food is required is a steam table. This device in its usual form comprises an elongated chamber having inserts provided in its top surface for the insertion of food-containing vessels especially made to fit into these inserts. The food containers receive heat carried by circulating steam and transmitted through the walls of the inserts. The temperature of the circulating steam is usually controlled by controlling of the pressure at which the steam is generated. (Steam which is generated at elevated temperatures and pressures is called super-heated steam.) The higher the pressures employed, the greater the danger of high pressure leaks and explosions with attendant danger to operating personnel. The circulating steam is usually transmitted to the steam table from a steam generator by means of high pressure hoses and fittings. Operation and maintenance of steam tables require specially trained personnel and specialized equipment, and maintenance and repair of these units are expensive and time-consuming. Moreover, since temperatures above boiling (212.degree.F) may be maintained only through the use of greater than atmospheric pressures, the steam table finds primary application in cooking at temperatures of around 212.degree.F or only slightly higher and in the maintenance of quantities of food at such temperatures for long periods of time.
Another cooking device commonly used in restaurants, hotels and other mass food processing establishments is the double boiler which consists of a vented chamber partly filled with a liquid working fluid, usually water. Application of heat to the double boiler causes its working fluid to boil and become vaporized at atmospheric pressure. The vaporized working fluid condenses on the inner working surfaces of the chamber causing its temperature to rise. Since the chamber is usually vented to the atmosphere, only one operating temperature may be maintained, that of the boiling point of the working fluid at the prevailing pressure. Thus, no temperature control is provided. Moreover, working fluid must be frequently added to the double boiler to replace the constant loss of fluid vapor to the atmosphere.
A cooking device which operates on the heat pipe principle is disclosed in U.S. Pat. No. 3,603,767 to Edward A. Scicchitano. This device consists of a fry pan having sealed evacuated reservoir disposed beneath the fry pan surface. A working fluid is disposed in the reservoir and, upon heating, forms vapor which condenses on the underside of the fry pan working surface and thereby heats the surface. The condensed vapor is drained from the underside of the surface by means of a capillary structure disposed in the reservoir. Heat control is afforded by means of a conventional thermostat control which plugs into the pan and thereby senses the pan temperature. A disadvantage of such an arrangement is that it senses a localized pan temperature. As a result, the application of cold food to the pan in the vicinity of the thermostat creates an apparent demand for additional applied heat which in turn causes overheating of the unused pan surface area away from the thermostat. Thus, the temperature control afford is relatively slow and inaccurate and nonuniform heating of the pan surface may result as cold food is placed on the pan. Moreover, overheating and overpressure form an ever-present danger which may lead to rupture of the device and attendant harm to the operator.